I'm not sure if there's a demo for the HD version. There is a demo for the normal version here on this page: Nifflas' NightSky Page

If you decide that it works well enough and you wanted to buy NightSky you can always buy it here on steam: NightSky on Steam

I personally don't know of a way to currently get NightSky's HD version. There was a beta, and I think it was released (before being polished, hence this bug-finding subforum) with a Humble Indie Bundle. Besides that, I don't know. Night Sky is already a wonderful game without the higher resolution graphics, though.